1. Field of the Invention
The present invention relates to a method for manufacturing an artificial grain boundary type Josephson junction device. More specifically, the present invention relates to a novel method for preparing an artificial grain boundary type Josephson junction device formed of an oxide superconducting thin film.
2. Description of Related Art
A Josephson junction device can be realized in various structures. Among the various structures, the most preferable structure in practice is a stacked junction realized by a thin insulating layer sandwiched between a pair of superconductors. However, Josephson junction devices, which are composed of a pair of superconductor regions weakly linked to each other by for example a point contact type junction, a Dayem bridge type junction, also exhibit Josephson effect, although they show different characteristics. In general, these Josephson junctions have fine structures in which the superconductor and/or the insulator are composed of thin films.
In order to realize, for example, a stacked type Josephson junction device by using an oxide superconductor, a first oxide superconducting thin film, an insulator thin film and a second oxide superconducting thin film are stacked on a substrate in the named order.
The thickness of the insulating layer of the stacked type Josephson junction device is determined by the coherence length of the superconductor. Since the coherence length of an oxide superconductor is very short, the thickness of the insulating layer must be about a few nanometers in the stacked type Josephson junction device formed of the oxide superconductor.
Further, both of the point contact type Josephson junction device and the Dayem bridge type Josephson junction device require a very fine processing such as a fine etching and a fine patterning, which makes it possible to realize a weak link between a pair of superconductors.
On the other hand, considering the operation characteristics of the Josephson junction device, each of the layers constituting the Josephson junction device has to have a high crystallinity and to be composed of a single crystal or a polycrystal having an orientation very close to that of a single crystal.
In the above mentioned stacked type Josephson junction device, therefore, it is necessary to stack a first oxide superconducting thin film, an insulator thin film and a second oxide superconducting thin film, which are of high crystallinity, respectively. However, it is difficult to stack an extremely thin and high crystalline insulator thin film on an oxide superconducting thin film. Furthermore, it is very difficult to stack a high crystalline oxide superconducting thin film on this insulator thin film because of the characteristics of the oxide superconductor.
Further, although the above mentioned stacked structure was realized, the interface between the oxide superconductor and the insulator was not in a good condition, so that a desired characteristics could not be obtained.
On the other hand, it is very difficult to conduct a fine processing such as a fine etching and a fine patterning on an oxide superconductor, which permits it to realize a point contact type Josephson junction device or a Dayem bridge type Josephson junction device. Therefore, a Josephson junction device using an oxide superconductor and having a stable performance could not be produced with good repeatability.
In view of the above mentioned problems, researches have been conducted for manufacturing a Josephson junction device taking advantage of the characteristics intrinsic to the oxide superconductor, while reducing the fine processing, such as a fine etching and a fine patterning, of the oxide superconductor to a possible extent. The oxide superconductor has a considerably different superconducting characteristics, dependently upon its crystalline direction. For example, if oxide superconductors having a crystalline direction different from each other are joined together, a grain boundary formed at the junction interface constitutes a barrier, so that a Josephson junction is formed. A Josephson junction device using this Josephson junction is called a artificial grain boundary type Josephson junction device, and can be manufactured without the fine processing as mentioned above.
As an example of the above mentioned artificial grain boundary type Josephson junction device, there may be mentioned a device formed by a junction between a c-axis oriented oxide superconducting thin film having its c-axis of crystal perpendicular to the substrate and another oxide superconducting thin film having its c-axis of crystal in parallel to the substrate (called "a-axis oriented oxide superconducting thin film" for the clarity of the description in this specification).
However, in order to manufacture a Josephson junction device comprising the artificial grain boundary having the structure as mentioned above, it is necessary to effect a processing of, for example, physically patterning a previously formed oxide superconducting thin film. In the course of this processing, the oxide superconducting thin film is exposed to the atmosphere so as to be degraded, with the result that an unnecessary Josephson junction is formed and/or a sharp grain boundary cannot be formed. Therefore, no process for manufacturing a Josephson junction device having a desired characteristics with good repeatability has been established yet.